The invention relates to an ozone generator, a metal electrode for use in an ozone generator, and methods of coating a metal electrode for use in an ozone generator. The invention is particularly applicable to tube type ozone generators.
Conventional tube type ozone generators use glass, quartz, or ceramic dielectrics. These dielectric materials have several drawbacks including relatively low dielectric strength (e.g. 300 volts/mil) and low dielectric constant (e.g. 6-10). The low electrical property values require that the dielectrics have a wall thickness between 3-5 millimeters to prevent electrical breakdown while sustaining the corona discharge necessary for ozone generation from oxygen containing gases. A thin layer of metallic silver or other low resistance material is applied to one side of the dielectric to serve as an electrical conductor.
The limited physical strength properties of these materials also require large supports. Although the ceramic materials have better electrical properties than the glass materials, physical strength property limitations require that the ceramics have even greater wall thickness than the glass material. The thick cross section and poor thermal conductivity of all these materials inhibits heat transfer. As heat is generated in these materials during ozone generator operation, the high temperature of the dielectrics causes thermal decomposition of the ozone being generated in the annular space between the electrodes of the ozone generator.
Titanium oxides have been used to replace glass dielectrics in ozone generators. Titanium oxide dielectrics are also far from ideal, however typically having a dielectric constant of about 80-170. There have been attempts to make higher dielectric strength dielectrodes for ozone generators using barium titanates since they can have dielectric constants of greater than 1,000. However such attempts have failed primarily because of the piezoelectric properties of barium titanates. When subjected to an electrical potential which applies an alternating current greater than 10,000 volts at a frequency of about 500 hertz (which is at the low end of typical for ozone generators), the dielectric coating typically ablates, cracks, or otherwise suffers damage as a result of such high voltage application over months of use.
According to the present invention, an ozone generator, and electrodes for an ozone generator, may be constructed which overcome the problems discussed above with respect to conventional electrodes in ozone generators. According to the invention, a dielectric is provided for an ozone generator which has a high dielectric constant, high dielectric strength, and low enough piezoelectric properties so that the dielectric will not ablate, crack, or otherwise suffer damage as a result of high voltage application even over months of use. The dielectrics according to the invention may have dielectric constants of greater than 1,000, like barium titanates, yet do not have the adverse piezoelectric properties thereof. The dielectrics preferably employed according to the present invention are known as PLZT mixed oxide ceramics, the designation "PLZT" referring to lead, lanthanum, zirconium and titanate. The concentration of individual components of such a mixed oxide ceramic affects the crystalline structure of the product as well as its electrical properties, and by varying the composition of the PLZT mixed oxide it is possible to make a material with a very high dielectric constant, high dielectric strength, and low piezoelectric activity.
According to one aspect of the present invention an ozone generator is provided comprising: First and second electrodes. Means for mounting the electrodes to define a flow path for oxygen containing gas between them. Means for applying an electrical potential to the electrodes sufficient to generate ozone from oxygen containing gas flowing in the flow path. And, a dielectric between at least one of the electrodes and the flow path, the dielectric comprising a mixed oxide composition having a dielectric constant of at least 200, a dielectric strength of at least about 800 volts/mil, and a low enough level of piezoelectric activity such that when the means for applying an electrical potential applies an alternating current greater than 10,000 volts at a frequency of about 500 hertz the dielectric coating will not ablate, crack, or otherwise suffer damage as a result of such high voltage application over months of use.
The first electrode may be a tube cantilevered at one end, and including another tube within the hollow interior thereof for circulating insulating coolant fluid (such as sulfur hexafluoride gas) therethrough. Typically both of the electrodes are tubular, and the cooperating surfaces of the tubes (inner or outer) are coated with the dielectric adjacent the gas flow path, although they need not be coated remote from the flow path. Either a conductive or a nonconductive cooling fluid may be circulated into operative association with the outer of the tubular electrodes.
The preferred dielectric coating according to the invention, for use on the electrode generator, on a metal surface thereof, has a composition comprising: about 30-70% lead oxide, about 2-8% barium oxide, about 2-12% lanthanum oxide, about 3-18% titanium dioxide, about 12-40% zirconium dioxide, and trace materials. The trace materials include silver, bismuth oxide, CdO, or combinations thereof. Dielectric constants of greater than 1,000 are relatively simple to obtain, in fact dielectric constants of well over 2,000 are practical.
A metal electrode can be coated a number of different ways for use in the ozone generator. For example according to one method the following steps are practiced substantially sequentially: (a) Mixing a plurality of different metal oxides together to form a mixed oxide composition. (b) Agglomerating the mixed oxide composition with about 1-5 weight percent binder and plasticizer combined to develop a particle size distribution suitable for plasma spraying. (c) Subjecting the agglomerated mixed oxide composition to treatment to provide a substantially uniform particle size. And, (d) plasma spraying the agglomerated mixed oxide composition substantially uniform particle size composition onto the metal electrode at a temperature high enough to substantially completely volatize the binder but low enough to substantially prevent volatization of low boiling point metals contained in the metal oxide mixture.
Step (d) is typically practiced at a temperature of about 1200.degree.-1300.degree. C., while step (a) is practiced to provide the composition set forth above. Step (c) is typically practiced by screening the agglomerated mixed oxide to remove particles having a maximum dimension above about 50 microns and below about 30 microns. Step (b) may be practiced utilizing polyvinyl alcohol as a binder and glycerol as a plasticizer.
According to another aspect of the present invention, a method of coating a metal electrode for use in an ozone generator comprises the steps of substantially sequentially: (a) Mixing a plurality of different metal oxides together to form a mixed oxide composition. (b) Stabilizing the mixed oxide composition in a volatile solution. (c) Applying the mixed oxide composition volatile solution to the surface of the metal electrode to achieve a desired coating thickness. And, (d) firing the coated electrode to sinter the mixed oxide composition.
Step (c) may be practiced by repeatedly dipping the electrode into the mixed oxide composition in a volatile solution, and air drying the coating formed after each dipping, until the desired thickness (typically less than 0.5 mm, e.g. 20 mils or less) has been obtained. Step (c) may alternatively be practiced by spin coating and then air drying the coating formed to the desired thickness after spin coating.
It is the primary object of the present invention to provide an ozone generator, and a metal electrode for use in an ozone generator, having a high dielectric strength, a very high dielectric constant, and low piezoelectric activity. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.